How to charge a laptop battery with a solar panel ?

Thread Starter


Joined Dec 29, 2008
I'm thinking of trying to charge my Lenovo Thinkpad X60s battery with an R-14 Powerfilm flexible solar panel.

The idea is to charge the battery with it in the laptop, but with the laptop turned off. I think having it turned On will most likely not work, for lack of supply power.

I think I have a basic Ohm's law type of problem - I've been told that since the wattage of the solar panel is much less than the draw of the Thinkpad, it won't work. I'm not an electrical engineer, although I did quite well in physics. The only problem is, it's been quite a few years (of disuse.)

Here are the numbers :

R-14 Powerfilm
The Powerfilm is rated at 15.4V and .9A ( ~14 watts)

Lenovo Thinkpad X60s Laptop

The battery is a lithion 14.4v / ~5200 mAh.
I'm not sure if the charge controller is internal or external to the battery, but it's within the confines of the laptop anyway.

I measured the current draw of the Thinkpad when plugged in - and when OFF (battery charging only) - at .25A and 115v (= 29watts ).
When On, it's something over 50 watts.
I measured this straight out of the wall, before the inline transformer that converts the 110v to ~20v (?). (I'm not sure what the charging voltage is, but the charger says 20v / 3.5A = 70watts - probably what's need to actually power a running computer)

I had thought that the battery charging would charge at a much lower current, like a trickle current, .1C or so, but this is not necessarily the case with Lithion batteries it seems. See Lithium Ion Battery Charging Basics
It's more like .2 to .7C actually.

One idea, if this is not going to work, is to pull the battery off and charge it outside the laptop, but this looks complicated as there are about 8 or 9 contacts on the battery, and I have no idea what is what. It sounds a little risky anyway.

If anyone could be of help that'd be great.
(I tried posting over at RFCafe, but so far not too much feedback.)

Last edited:


Joined Aug 7, 2008
I would use the internal charger with an external supply, AC or DC if comp. has full-wave bridge rectifier. Use a DC boost converter to raise panel output to 20V.


Joined May 26, 2009
TI, Analog Devices, Linear Tech, and a few other companies have DC-DC converters that can step the voltage up real nicely...although I'm not sure how well they do considering the current; but you could have a current regulator to provide more current.

Thread Starter


Joined Dec 29, 2008
Ok, thanks for the advice.

I'm still wondering a bit about my basic question though . . .

Am I not going to be able to charge a battery that draws 29+ watts, with a solar panel that can only deliver 14 watts ?

The panel is pretty large, physically, which is what gave me the idea, but I'm thinking that the internal controller will not receive enough current even if I'm able to boost the voltage to ~20v. Or will it just charge at a slower rate. I guess I don't know enough about what these internal controllers require. All I know from my link on lithion charging above is that it's not a good idea to charge at too low a voltage.